This invention relates to a traveling wave tube device, more particularly to an improvement of a periodic permanent magnet assembly of a traveling wave tube device.
As is well known in the art, the traveling wave tube device of the type referred to above comprises a pumped envelope made of such insulating material as glass or ceramic and containing therein an electron gun for emitting an electron beam, a helical delay line or helix for causing the electron beam to interact with a microwave power amplified, and a collector for collecting the electron beam passed through the helix; and a periodic permanent magnet assembly coaxially surrounding a portion of the envelope which contains the helix and support rods thereof.
The periodic permanent magnet assembly comprises means for input and output microwave power to and from the traveling wave tube, a plurality of cylindrical permanent magnets which are magnetized in the axial direction and disposed between the input/output means, and alternately disposed pole pieces having an inner diameter smaller than that of the permanent magnets, thereby producing periodic magnetic field for focusing the electron beam along the tube axis. Accordingly, the electron beam emitted by the electron gun travels through a path inside of the helix while being focused by the periodic permanent magnet assembly.
Meanwhile, the microwave power supplied to the input end of the helix via the input device is amplified by an energy coupling with the electron beam traveling through the inner space and then taken out as an amplified microwave power through the output end of the helix.
The microwave power output characteristics of this type of the traveling wave tube device are generally limited by the following two reasons.
One reason lies in the heat generated mainly on the output side of the helix due to the amplification of the microwave power. This heat generates unwanted gas in the tube or renders unstable the microwave output power due to temperature variation.
The amount of heat thus generated increases with the microwave power level thereby increasing the amount of the gas generated and instability of the microwave output power. Further, these phenomena limits the permissible microwave power of the helical delay line thus making it impossible to produce a large microwave power output.
In addition, when the microwave output power becomes unstable in a manner as above described, various limits would be imposed upon microwave communication systems utilizing the traveling wave device.
One solution of these problems is disclosed in Japanese utility model publication No. 21478/1978 dated June 5, 1978.
According to this utility model publication there is provided an expandable and contractable ring shaped electroconductive member which is fitted to the inner surfaces of the pole pieces of the periodic permanent magnet assembly, the outer surface of the electroconductive member being in contact with the permanent magnets and the inner surface being secured to the outer surface of the helix portion envelope. With this construction, the heat generated by the helix is conducted to the helix portion envelope through dielectric members provided for the purpose of supporting the helix, then to the permanent magnets and their pole pieces through the electroconductive members and finally dissipated to surrounding air.
With this construction since the heat is dissipated by the path described above, the cooling effect is improved over the prior art construction thus increasing the permissible heat capacity caused microwave power of the helix.
As the material for preparing the expandable and contractable electroconductive member is used a silicone rubber mixed with a powder of silver, or a troidal coil shaped ring is used.
The silicone rubber incorporated with a powder of silver is difficult to manifest uniform electroconductivity which is essential to the traveling wave tube device thus causing nonuniform heat radiation. A fatal defect of this construction lies in that the silicone rubber becomes hard and brittle due to aging thus losing the resiliency as well the heat conductivity.
Since the troidal coil shaped ring is wound one turn about the helix portion envelope, the problems caused by the silicone rubber would not be resulted. But as the contact areas between the envelope and the ring and between the ring and the pole pieces and the permanent magnets are small the heat conductivity is smaller than the construction utilizing the silicone rubber thus greatly reducing the cooling effect. Where a single troidal coil shaped ring is used undesirable phenomenon occurs wherein the microwave power is fed back to the input side from the output side of the microwave power amplifier device.
Moreover, the permissible heat capacity to the helix can not be increased in any appreciable extent because the cooling effect is not improved as exposed.